This invention concerns a train detection mechanism and method for detecting the presence of a train on a section of track.
It is common to determine the presence of a train on a section of track by using a track circuit. Conventionally the track circuits may be arranged in two fundamental ways, those where the transmitter and receiver are located at different positions within the track circuit and those where the transmitter and receiver are co-incident at one end of the track circuit. With the co-incident method, current is injected into one of the rails. Normally, i.e. in the absence of a train, the current will flow along the rail, pass through a shunt provided to connect the rails and then return back along the second rail. The shunt device presents a low impedance to the current and may consist of a simple wire bond between the rails where no other a.c. or d.c. is present on the rails. A narrow band shunt is used where a.c. is present and a wide band shunt is used where non-coded d.c. is present. The current across both rails may thus be measured, and hence the impedance of the circuit. If a train is present in the track section, the current may pass from one rail to the other through the wheels and an axle of the train. As the train passes along the track section, the length of the circuit will change, and the impedance of the circuit will also correspondingly change. By measuring this change in impedance, it is therefore possible to determine not just that a train is present on the track section, but also the speed and hence position of the train within the section.
Track circuits such as these may be used anywhere in a rail system. For example, a pair of track circuits may be used to determine the presence of a train in the run-up to a level crossing, i.e. one track circuit is used on each side of the level crossing, to detect trains approaching from either direction, to cater for single lines and bi-directional traffic. Such an arrangement is shown in FIG. 1, where a track circuit is shown on each side of a central level crossing island. In this instance the audio frequency current is injected and subsequently received by a transmitter/receiver module in a Grade Crossing Predictor (GCP) via a termination shunt. The GCP is a microprocessor-controlled device that provides activation of the crossing protection equipment, such as warning lights and barriers, by sensing the approach of a train. The GCP is located at the crossing whilst the termination shunt is fitted across the rails at a suitable distance from the crossing to define the limit of the track circuit. This limit is chosen to provide sufficient warning time for the fastest train that may be encountered. The GCP applies a constant current a.c. signal to the track and measures the level of the resulting voltage.
As a train approaches the crossing, the impedance of the track circuit changes once the train passes over the shunt, and continues to change as the train moves closer. This change of impedance is constantly monitored by the GCP via voltage variation and, by calculating the rate of change, the speed of the train is determined. From this speed, the moment at which the crossing needs to be activated is determined and the warning is given and the road closed to traffic accordingly. By this means, a constant warning time can be achieved regardless of the variety of train speeds that may be encountered.
However, detection of the train relies upon achieving a wheel to rail interface of sufficiently low resistance to effect a path for the track circuit current. Conditions of rail head contamination, or the light weight of some vehicles can result in a wheel to rail interface that is of too high a resistance to achieve activation.